In the last year of the Third Age [Éomer Éadig] wedded Lothíriel, daughter of Imrahil. Their son Elfwine the Fair ruled after him.

The Lord of the Rings, Appendix A, Annals of the Kings and Rulers: The House of Eorl

[Imrahil's] kinship1 with Éomer of Rohan, though distant, was recognized by Imrahil, and great friendship grew between them. Éomer wedded Imrahil's daughter, and their son, Elfwine the Fair, had a striking likeness to his mother's father.

1Éomer's maternal grandmother, Morwen of Lossarnach, was descended from the line of Princes of Dol Amroth:

Éomer... with other descendants of King Thengel were taller than the norm of Rohan, deriving this characteristic (together in some cases with darker hair) from Morwen, Thengel's wife, a lady of Gondor of high Númenórean descent....

She was known as Morwen of Lossarnach, for she dwelt there; but she did not belong to the people of that land. Her father had removed thither, for love of its flowering vales, from Belfalas; he was a descendant of a former prince of that fief, and thus a kinsman of Prince Imrahil.